For many years it has been known that aqueous zinc phosphate coatings when applied to metallic surfaces such as steel, galvanized iron and aluminum form conversion coatings which provide corrosion resistant properties to such metallic surfaces and likewise provide a surface to which siccative coatings such as paint will adhere. However, in preparing conversion coatings for a paint application, attempts to apply such conversion coatings by immersion of the metal surfaces in a zinc phosphate bath have not been satisfactory due to the tendency under such conditions to form heavy, coarse coatings. Consequently, the art has turned to the use of spray processes to obtain finer coatings. Such spray processes have the disadvantage of a lesser penetration of crevices and inaccessible areas as compared to immersion processes but the coatings thus obtained are noticeably finer although still in need of improvement, particularly as to corrosion resistance. Another disadvantage of the processes of the art is the difficulty in coating the areas of steel surfaces upon which there lies carbon residue. Such areas of carbon residue on steel surfaces are common occurrences.
When used as a base for paint, the process of the present invention provides a fine-grained smooth, uniform and penetrating conversion coating on metallic surfaces using a zinc phosphate immersion process, thus overcoming the disadvantages inherent in the prior immersion processes and at the same time overcoming the disadvantages inherent in the prior spray processes.
In preparing zinc phosphate coatings suitable for application of rust preventive oil, attempts to apply such conversion coatings by immersion of the metal surfaces in a zinc phosphate bath have not produced optimal results due to the tendency under such conditions to form a coating which is coarse and granular and does not provide an optimal base for rust preventive oil.
The present invention provides finer crystal structure and covers the metallic surface, including crevices, more completely, thereby providing greater rust protection than previously possible. The present invention also provides the capability of covering the areas of carbon residue found on steel surfaces.